The acquisition and adaptation of perceptual-motor is fundamental to everyday life. In the inquiry of skill acquisition and control the concepts of stability and variability have played a central role, albeit with different definitions and levels of rigor. Most commonly, improvement of performance has been associated with a decrease in variability of some task parameters. This reduced variability, in turn, has been interpreted as an increase in stability. This inverse relationship obscures that empirical variability can be indicative of many different facets, ranging from the obvious "lack of control", seen as errors or inconstancy in target-oriented tasks, to more beneficial aspects, such as compensatory variation between parameters, and exploration of new tasks. Abnormal levels of variability are characteristic for dysfunctional behavior, as evidenced in tremor, excessive movements, or stereotypy, the absence of fluctuations. A fundamental assumption in our approach is that a certain irreducible level of "noise" is a basic expression of biological systems. Hence, also stability is more than invariance of some performance parameter. Specifically, dynamical stability is defined as resistance to perturbations, which can be quantified independently from variability. The project examines skill acquisition in two tasks to differentiate our understanding of variability and stability in human action. In skittles, a target-oriented throwing action under feedforward control, we develop a method to decompose variability into three independent components: tolerance, covariation, and noise, each capturing a separate contribution to successful behavior. In 6 experiments we test how different components of variability contribute in different stages of learning, and how stochastic noise can be a means to find successful solutions. The second task is the continuous perceptually-guided skill of rhythmically bouncing a ball. Previous modeling work derived criteria for dynamically stable performance and we showed that subjects used this "passive" stability. Six experiments examine how skill acquisition is characterized by an increasing reliance on dynamical stability. Performance variability will be analyzed to test how different components contribute to this change in stability. Obtaining a deeper understanding of acquisition and the control of perceptual-motor tasks will advance knowledge for diagnosis of movement disorders as well as to develop methods of rehabilitation. [unreadable] [unreadable]